New polymer compositions having properties that are particularly tailored for specific applications are required in response to more sophisticated purchasers of polymers. It is extremely costly and/or difficult to make these compositions directly by synthesis or via solution esterification or transesterification, but manufacturing them in mixing equipment such as an extruder has provided an economical and viable means to supply increasingly-complex polymers to specialized markets requiring them.
In one embodiment, this invention provides specialty polymers whose compositions have a polyethylenic backbone and pendant benzyl ester moieties. In another embodiment, this invention provides compositions having a polyethylenic backbone and pendant hydrocarbyl ester moieties that contain at least one allylic hydrogen. In a further embodiment, this invention provides compositions having a polyethylenic backbone and pendant hydrocarbyl moieties, especially hydrocarbyl ester moieties, wherein the hyrocarbyl group contains a heteroatom such as oxygen. In a preferred embodiment, this invention provides compositions having a polyethylenic backbone and pendant ether ester moieties, especially cyclic ether ester moieties. In certain preferred embodiments, the compositions of this invention have the following structures: ##STR1## where n is an integer from 2 to approximately 30,000; any X is individually chosen from the group consisting of hydrogen and methyl radical; and where any Y is individually chosen from the group consisting of hydrogen, alkyl radicals containing from 1 to 18 carbon atoms, alkoxy radicals having from 1 to 16 carbon atoms, alkyl ether radicals having from 2 to 18 carbon atoms, alkenyl and alkynyl radicals containing from 2 to 18 carbon atoms, alkenoxy and alkynoxy radicals having from 2 to 16 carbon atoms, alkenyl and alkynyl ether radicals having from 3 to 18 carbon atoms, amine radicals having from 1 to 16 carbon atoms, acid and metal salt of acid radicals, anhydride radicals having from 4 to 24 carbon atoms, ester and amide radicals of acids having from 1 to 16 carbon atoms, aryl radicals and substituted aryl radicals having 6 to 24 carbon atoms, aryl ether radicals and substituted aryl ether radicals having from 6 to 24 carbon atoms, and the radicals of Formula II and Formula III ##STR2## where any A is individually a heteroatom-containing radical (especially a carboxy or amido), and where any B is individually chosen from the group consisting of alkyl ether radicals having from 2 to 18 carbon atoms, alkenyl and alkynyl radicals containing from 2 to 18 carbon atoms, alkenoxy and alkynoxy radicals having from 2 to 16 carbon atoms, alkenyl and alkynyl ether radicals having from 3 to 18 carbon atoms, substituted aryl radicals having 6 to 24 carbon atoms, aryl ether radicals and substituted aryl ether radicals having from 6 to 24 carbon atoms, and the radicals of Formula IV and Formula V: ##STR3## where any R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, and R.sup.6 is individually chosen from the group consisting of hydrogen, alkyl radicals containing from 1 to 18 carbon atoms, alkoxy radicals having from 1 to 16 carbon atoms, amine radicals having from 1 to 6 carbon atoms, ester and amide radicals of acids having from 1 to 16 carbon atoms, aryl radicals and substituted aryl radicals having 6 to 24 carbon atoms, aryl ether radicals and substituted aryl ether radicals having from 6 to 24 carbon atoms, and the radicals of Formula II and Formula III; with the proviso that at least about 1 mole % of the composition comprises the radicals of Formula II and Formula III.
These specialty polymers are useful as packaging films and are also useful components for making oxygen-scavenging compositions.
This invention also provides new oxygen-scavenging compositions. It is well known that regulating the exposure of oxygen-sensitive products to oxygen maintains and enhances the quality and "shelf-life" of the product. For instance, by limiting the exposure of oxygen sensitive food products to oxygen in a packaging system, the quality or freshness of food is maintained, and the food doesn't spoil as rapidly. In addition, oxygen-scavenging packaging also keeps the product in inventory longer, thereby reducing costs incurred from waste and having to restock inventory. In the food packaging industry, several means for regulating oxygen exposure have already been developed. These means include modified atmosphere packaging (MAP) and oxygen barrier film packaging.
One method currently being used is through "active packaging", whereby the package containing the food product has been modified in some manner to regulate the food's exposure to oxygen. One form of active packaging uses oxygen-scavenging sachets which contain a composition which scavenges the oxygen through oxidation reactions. One type of sachet contains iron-based compositions which oxidize to their ferric states. Another type of sachet contains unsaturated fatty acid salts on a particulate adsorbent. See U.S. Pat. No. 4,908,151. Yet another sachet contains metal/polyamide complex. See U.S. Pat. No. 5,194,478.
However, one disadvantage of sachets is the need for additional packaging operations to add the sachet to each package. A further disadvantage arising from the iron-based sachets is that certain atmospheric conditions (e.g., high humidity, low CO.sub.2 level) in the package are sometimes required in order for scavenging to occur at an adequate rate. Further, the sachets can present a danger to consumers if accidentally ingested.
Another means for regulating exposure of a packaged product to oxygen involves incorporating an oxygen scavenger into the packaging structure itself. A more uniform scavenging effect throughout the package is achieved by incorporating the scavenging material in the package instead of adding a separate scavenger structure (e.g., a sachet) to the package. This may be especially important where there is restricted air flow inside the package. In addition, incorporating the oxygen scavenger into the package structure provides a means of intercepting and scavenging oxygen as it permeates the walls of the package (herein referred to as an "active oxygen barrier"), thereby maintaining the lowest possible oxygen level in the package.
One attempt to prepare an oxygen-scavenging wall involves the incorporation of inorganic powders and/or salts. See U.S. Pat. Nos. 5,153,038, 5,116,660, 5,143,769, and 5,089,323. However, incorporation of these powders and/or salts causes degradation of the wall's transparency and mechanical properties such as tear strength. In addition, these compounds can lead to processing difficulties, especially when fabricating thin films. The oxidation products, which can be absorbed by food in the container, typically would not have FDA approval for human consumption.
EP 0 519 616 discloses an oxygen-scavenging composition comprising a blend of a first polymeric component comprising a polyolefin, the first polymeric component having been grafted with an unsaturated carboxylic anhydride or an unsaturated carboxylic acid, or combinations thereof, or with an epoxide; a second polymeric component having OH, SH, or NHR.sup.2 groups where R.sup.2 is H, C.sub.1 -C.sub.3 alky, substituted C.sub.1 -C.sub.3 alkyl; and a metal salt capable of catalyzing the reaction between oxygen and the second polymeric component, the polyolefin being present in an amount sufficient so that the blend is non phase-separated. A blend of polymers is utilized to obtain oxygen scavenging, and the second polymeric component is preferably a polyamide or a copolyamide such as the copolymer of m-xylylene-diamine and adipic acid (MXD6).
The oxygen scavenging systems disclosed in U.S. Pat. Nos. 5,021,515, 5,194,478, and 5,159,005, European Publication EP 0 380 319 as well as PCT Publication Nos. 90/00504 and 90/00578 illustrate attempts to produce an oxygen-scavenging wall. These patent applications disclose incorporating a metal catalyst-polyamide oxygen scavenging system into the package wall. Through catalyzed oxidation of the polyamide, the package wall regulates the amount of oxygen which reaches the interior volume of the package (active oxygen barrier) and has been reported to have oxygen scavenging rate capabilities up to about 5 cubic centimeters (cc) oxygen per square meter per day at ambient conditions. However, this system suffers from significant disadvantages.
One particularly limiting disadvantage of polyamide/catalyst materials can be a low oxygen scavenging rate. U.S. Pat. No. 5,021,515, Example 7, illustrates that adding these materials to a high-barrier package containing air produces a package which is not generally suitable for creating an internal oxygen level of less than 0.1% (starting with air) within a period of four weeks or less at room temperature, as is typically required for headspace oxygen scavenging applications.
There are also disadvantages to having the oxygen-scavenging groups in the backbone or network structure in this type of polyamide polymer. The basic polymer structure degrades rapidly and is quickly weakened upon reaction with oxygen. This can adversely affect physical properties such as tensile or impact strength of the polymer. The degradation of the backbone or network of the polymer can increase the permeability of the polymer to those materials sought to be excluded, such as oxygen.
Moreover, polyamides such as MXD6 are typically incompatible with thermoplastic polymers used in flexible packaging walls, such as ethylene-vinyl acetate copolymers and low density polyethylene. Even further, when many polyamides are used by themselves to make a flexible package wall, they may result in inappropriately stiff structures. Many polyamides also incur processing difficulties and higher costs when compared with the costs of thermoplastic polymers typically used to make flexible packaging. Even further, they are sometimes difficult to heat seal. Thus, all of these are factors to consider when selecting materials for packages, especially flexible packages and when selecting systems for reducing oxygen exposure of packaged products.
Another approach to scavenging oxygen is disclosed in EP 0 507 207, which discloses an oxygen-scavenging composition comprising an ethylenically unsaturated hydrocarbon and a transition metal catalyst. This patent states that ethylenically unsaturated compounds such as squalene, dehydrated castor oil, and 1,2-polybutadiene are useful oxygen scavenging compositions, and ethylenically saturated compounds such as polyethylene and ethylene copolymers are used as diluents. Compositions utilizing squalene, castor oil, or other such unsaturated hydrocarbon typically have an oily texture, which is undesirable for applications such as wrapping meat for sale in retail grocery stores. Further, polymer chains which are ethylenically unsaturated would be expected to either cross-link to become brittle or to degrade upon scavenging oxygen, weakening the polymer due to polymer backbone breakage.
U.S. Pat. Nos. 4,717,759, 4,994,539, and 4,736,007, which are incorporated by reference in their entirety, disclose ethylene copolymers which comprise 85.0 to 99.995 mol % of an ethylene unit, 0.005 to 5 mol % of a comonomer unit represented by Formula (VI) ##STR4## wherein Ar is ##STR5## R.sub.1 is a hydrogen atom or a methyl group, each of R.sub.2 and R.sub.3 is a hydrogen atom, a chlorine atom or a straight-chain or a side-chain alkyl group having 1 to 4 carbon atoms, and 0 to 10 mol % of an ethylenic unsaturated monomer unit, the ethylene copolymer having a density of 0.860 to 0.970 g/cm.sup.3 and a melt index of 0.05 to 100 g/10 minutes. The patent states that copolymers may be produced using either a Ziegler catalyst or through polymerization catalyzed by free radicals. These polymers are limited to having less than 5 mol % of the comonomer unit and are useful for electrical insulation. Although these polymers may be used to make oxygen-scavenging compositions, these polymers do not themselves scavenge oxygen.
What has been needed is an oxygen-scavenging polymer composition that is easily processed, especially into thin film, and that does not suffer rapid polymer backbone oxidation as the composition scavenges oxygen. This invention supplies compositions overcoming these problems.
The oxygen-scavenging compositions of this invention comprise a transition-metal salt and a compound having an ethylenic or polyethylenic backbone and having pendant or terminal moieties which contain a carbon atom that can form a free radical that is resonance-stabilized by an adjacent group. Thus, a carbon atom having a hydrogen atom adjacent to a phenyl radical, an ethylenically-unsaturated carbon atom, or a heteroatom such as oxygen can form a free radical that is resonance-stabilized by the adjacent double bond, phenyl ring, or oxygen, respectively.
In one embodiment, the invention provides a composition comprising a transition-metal salt and a component having the structure of Formula (I) above. The invention also provides new compositions comprising a transition-metal salt and a polymer which comprises a polyethylenic backbone and a pendant moiety comprising a benzyl radical having at least one hydrogen atom on the methylene group of the benzyl radical, and/or an allylic radical and/or an ether radical that individually contain at least one hydrogen atom alpha to these radicals.
In another embodiment, the invention provides a composition comprising a transition-metal salt and a polymer, where said polymer comprises 1) a polyethylenic backbone, and 2) pendant moieties which have at least one radical selected from the group consisting of a) benzyl ester radicals, b) N-benzyl-amide radicals, c) N-benzylimide radicals, d) benzyl-thio radicals, e) benzyl ketone radicals, f) benzyl-ether radicals, g) aryl radicals and substituted aryl radicals having 6 to 30 carbon atoms, h) aryl ether radicals and substituted aryl ether radicals having from 6 to 30 carbon atoms, and i) benzyl radicals which have the phenyl radical of said benzyl radical chemically bonded to at least one member selected from the group consisting of imide radicals which are N-substituted with said benzyl radicals, benzyl-ketone radicals, alkyl radicals containing from 1 to 18 carbon atoms, alkoxy radicals having from 1 to 16 carbon atoms, amine radicals having from 1 to 6 carbon atoms, ester and amide radicals of acids, said ester and amide radicals having from 1 to 16 carbon atoms, aryl radicals and substituted aryl radicals having 6 to 24 carbon atoms, and aryl ether radicals and substituted aryl ether radicals having from 6 to 24 carbon atoms.
In another embodiment, the invention provides a composition comprising a transition-metal salt and an ethylenic or polyethylenic backbone having a pendant or terminal benzyl radical, wherein the composition, upon reaction with molecular oxygen, produces benzoic acid or a benzoic acid substituted with at least one radical selected from the group consisting of alkyl radicals containing from 1 to 18 carbon atoms, alkoxy radicals having from 1 to 16 carbon atoms, amine radicals having from 1 to 6 carbon atoms, ester and amide radicals of acids having from 1 to 16 carbon atoms, aryl radicals and substituted aryl radicals having 6 to 24 carbon atoms, and aryl ether radicals and substituted aryl ether radicals having from 6 to 24 carbon atoms.
The invention also provides a composition comprising an ethylenic or polyethylenic backbone and moieties which contain a radical having an allylic hydrogen and which are pendant or terminal to the ethylenic or polyethylenic backbone. The radical containing allylic hydrogen may be cyclic, linear, or branched, and may be unsubstituted or substituted with alkyl, aryl, or heteroatom-containing radicals, for example.
The invention also provides a composition comprising an ethylenic or polyethylenic backbone and hydrocarbyl moieties which contain at least one hydrogen alpha to an ether radical and which are pendant or terminal to the ethylenic or polyethylenic backbone. The moiety containing ether may be cyclic, linear, or branched, and may be unsubstituted or substituted with alkyl, aryl, or other radicals containing a heteroatom, for example.
Among other factors, the present invention is based on our finding that compositions as described herein are highly effective oxygen scavengers in terms of rate of oxygen scavenging and/or oxygen scavenging capacity, particularly where the compositions contain a heteroatom-containing radical such as a carboxy or amido group directly bonded to a benzyl radical, an allylic radical, or an ether radical. In many instances, these compositions have excellent physical and processing properties which permit their incorporation into a wide range of packaging applications. We have found that, typically, films of these compositions are easily made using conventional techniques. The compositions are usually compatible with many common thermoplastic materials used in packaging, particularly polyethylene and copolymers of ethylene and alkyl acrylates or methacrylates.
Furthermore, many of the compositions of the present invention have been found to have surprisingly reduced induction periods in scavenging oxygen upon exposure to ultraviolet (UV) radiation without the need for added photo-initiators.
This invention also provides a process for making polymers, some of which are useful in making oxygen scavenging compositions. Transesterification of a polymer can produce a number of different polymers. For example, M. Lambla et al., 27 Polymer Sci. and Eng'g, No. 16 (mid-Sept. 1987) 1221-28, discuss the transesterification of ethylene vinyl acetate copolymer with an alcohol in an extruder and in the presence of a tin catalyst to form ethylene vinyl alcohol copolymer, which has a polyethylenic backbone and pendant alcohol moieties. Also, D. Seebach et al., Synthesis (Feb. 1982) 138-41, discuss transesterification of an ester with an alcohol in solution using a titanium catalyst. The reactions require from 3 to 120 hours.
U.S. Pat. No. 4,767,820 to M. Keogh discloses compositions useful as extrudates about wires and cables which comprise hydrolyzable pendant silane moieties and tetramethyl titanate dispersed in a normally solid alkylene-alkyl acrylate copolymer matrix. Transalkylation of the silane and alkyl acrylate moieties results in a cross-linked product.
Not all transesterification reactions are useful. U.S. Pat. No. 5,023,284 to M. Cheung et al. notes that transesterification occurs during melt-blending of two polyesters due to the presence of residual titanium catalyst and causes embrittlement and other deleterious effects.
What has been missing in the prior art is an economical process for controlling the esterification and/or transesterification of a polymer having a polyethylenic backbone and pendant acid and/or ester moieties to produce a polymer having a polyethylenic backbone and pendant ester moieties that differ in number and/or type from the unreacted polymer. In one embodiment, this invention provides an economical process for esterifying or transesterifying a polymer comprising forming a melt of a polymer having a polyethylenic backbone and pendant acid or ester moieties, and contacting the melt in suitable mixing equipment (for example, an extruder) under esterification or transesterification conditions with a compound capable of esterifying or transesterifying the acid or ester moieties, where the polymer undergoes esterification and/or transesterification but not alcoholysis, and the polymer after esterification and/or transesterification has a polyethylenic backbone and pendant ester moieties.
The process may further comprise adding an amount of transition metal salt into a melt of selected esterified or transesterified polymers made in the above process in an amount effective to promote oxygen scavenging by the esterified or transesterified polymer. In one preferred embodiment, an ethylene alkyl acrylate copolymer is transesterified in an extruder to form an ethylene hydrocarbyl acrylate copolymer. In another preferred embodiment of the process, a cobalt salt is added to the transesterified polymer to make an effective oxygen scavenger. In a third preferred embodiment, the processed polymer is exposed to actinic radiation.
Among other factors, it has been discovered that a melt of a polymer having pendant acid and/or ester moieties can be esterified and/or transesterified with a compound capable of esterifying or transesterifying the acid and/or ester moieties by blending the melt and the compound in suitable polymer mixing equipment under esterification and/or transesterification conditions, thereby producing a polymer having pendant ester moieties which differ in number and/or type from the unreacted polymer. This process provides fast reaction times and accurate control over the extent of esterification and/or transesterification, thereby providing an economical means to produce polymers having properties tailored to specific applications. The process also provides a means to make highly-effective oxygen scavenging compositions.
This invention also provides compositions that can be made by the process of esterifying or transesterifying a melt of a polymer having an ethylenic or polyethylenic backbone. The above-mentioned advantages and others are further described below.